As Boston reels from the Marathon bombings, more than 100 investigators from Boston police, state, and federal agencies are combing Boylston Street, conducting scores of interviews and scouring ­videos and photographs in a painstaking attempt to reconstruct a scene that took mere seconds to blow apart. Building an accurate picture could take months. But investigators must work quickly to gather much of the evidence before the wind or rain carries it away.

“We are taking evidence off the tops of buildings,” said one law enforcement official. “We are going to need at least ­another day before we can open up the area.”

Forensic scientists say the first few hours of an investigation are crucial, especially ­because bad weather could damage the evidence. Investigators on the scene wore protective gear, which scientists say is meant to prevent their DNA from mixing with the evidence. As they search, investigators are attempting to gather information that will answer questions, such as: What kind of bomb is it? How was the bomb triggered? Was it in a container? Is there any DNA or fingerprints on the pieces?

“Bomb scenes are very complicated because the stuff blows all over the place,” said Barry Fisher, a retired crime lab director for the Los Angeles County Sheriff’s Office. “They’re going to want to go and pick up as much of the ­debris as possible because there may be some evidence that will help them decipher what happened.”

By Tuesday afternoon, inves­tigators had a major breakthrough when they discovered a circuit board they ­believe was used to detonate the bombs. Investigators also recovered pieces of the bombs, and an official said the bombs had been placed in black nylon bags or backpacks.

Investigators also fanned out across Boston to secure what could be crucial pieces of evidence.

In operating rooms at Tufts Medical Center and elsewhere, surgeons removed pellets, nails, and ball bearings from victims, carefully placed them into labeled specimen containers and handed them over to police.

At Logan International Airport, Homeland Security officials stopped departing passengers and asked them to share any videos or photographs from the finish line at the Marathon.

Law enforcement officials are poring over hundreds of videos and photographs from security cameras and Marathon-­goers to identify people in the area. Information from the scene can help narrow their searches, such as locat­ing a person who might have been carrying the type of bag the devices were found in.

“You never know what ­image or what video is going to provide a needed clue,” said Jarrad R. Wagner, associate professor of forensic sciences at Oklahoma State University and a former FBI chemist.

Finding answers can be formidable because the crime scene includes the final stretch of the
Marathon, an area traversed by thousands of people from all over the world. And since Boylston Street is one of the city’s main thoroughfares, investigators can keep it closed off for only so long.

“It is very difficult,” said Wagner. “On TV, it’s always so simple. When you go into these scenes they’re chaotic. They’re very large scenes. There’s so much to collect. . . . Your fear is: ‘Am I getting it? Am I collecting? Did I miss something?’ ”

At a press conference Tuesday morning, Boston Police Commissioner Edward F. Davis urged the public to have ­patience with investigators as they secure the site at the ­direction of the FBI and the ATF, calling it “the most complex crime scene we’ve dealt with in the history of our depart­ment.” Davis predicted the scene could be closed off for “a couple of days.”

Forensic scientists said each new discovery could help inves­tigators trace those respon­sible and lay the trail for a successful prosecution.

Wagner, for instance, said investigators are probably checking the streets for anything that might reveal how the bomb was triggered.

He said it was unlikely the bombers lit a fuse, as Timothy McVeigh did in April 1995 in the blasts that killed 168 people in Oklahoma City.

“I don’t think the bag could sit there for that long with smoke emanating and have no one notice it at the Boston Marathon,” he said.

Bruce Goldberger, director of forensic medicine at the University of Florida and a past president of the American Academy of Forensic Sciences, said much of the evidence could now be in small pieces, making the work on Boylston Street all the more important.

“Right now they have to follow every individual, minute lead,” he said.

“You can’t determine immediately whether it’s an important piece of evidence or evidence that can be discarded. That’s why Boylston Street is closed.”